Large deposits of heavy oils exist in the near surface in many places, including, e.g., Canada and South America. Certain heavy oil deposits are referred to variously as sand oils, tar sands and bitumen. These heavy oil deposits are often located close to the surface of the Earth. However, at natural ambient temperatures in the subsurface these heavy oils often cannot be extracted by conventional means, because the viscosity of these hydrocarbons is typically too high to enable fluid flow at ambient near surface Earth temperatures.
There also exist many locations where hydrocarbons are present as contaminants in the shallow subsurface. In some cases these hydrocarbons may be introduced to the subsurface by accidental spills or leaks in underground storage tanks. Removing these contaminants can be challenging because they are not in the aqueous phase and may have viscosity and wetting properties that differ from groundwater and may prevent the contaminant from flowing into wells.
Various induced alteration processes are sometimes used to reduce the viscosity and/or change other properties of hydrocarbons so they can be produced or otherwise removed from a subsurface formation. Such techniques may include stimulating a reservoir formation thermally with heat or chemically with solvents, and may be generally referred to generally as heavy oil “stimulation” or “enhanced oil recovery (EOR)” techniques. Thermal recovery stimulation may include, for example, the use of steam, as in the established method of steam-injection and steam assisted gravity drainage (known by the acronym SAGD). Thermal stimulation may alternatively or additionally include in-situ combustion of hydrocarbons and microwave heating. Chemical recovery stimulation methods include, for example, vapor extraction, known by the acronym VAPEX.
Accurate measurement is useful in selecting heavy oil reservoirs as well as optimizing EOR operations in heavy oil reservoirs or contaminated sites. As a result, there is an ongoing need in the industry to improve measurement techniques, whether by improving measurement accuracy, improving measurement timing and frequency, increasing the different types of data that can be measured, improving the ease of making measurements, improving the cost of making measurements, increasing the different conditions under which measurements can be made, or otherwise.
Nuclear Magnetic Resonance (NMR) systems have been in use for many years and can be used to provide imaging and/or analysis of a sample being tested. For example, U.S. Pat. No. 6,160,398, U.S. Pat. No. 7,466,128, U.S. Pat. No. 7,986,143, U.S. patent application Ser. No. 12/914,138, and U.S. patent application Ser. No. 13/104,721 describe a variety of NMR technologies, and are incorporated herein by reference. Various different types of NMR include medical NMR, often referred to as magnetic resonance imaging (MRI), and geophysical NMR for measuring properties of Earth formations, including surface NMR and logging NMR. While there is some overlap in the technologies that may be applied in MRI and geophysical NMR, the samples being measured and the environments in which measurements are performed are different, leading to many differences in the technologies applied.
In general, NMR measurement involves utilizing or generating a static magnetic field within a sample volume, emitting one or more electromagnetic pulses into the sample volume, and detecting NMR responses from the sample volume. In some cases, NMR measurement involves emitting multiple electromagnetic pulses in rapid succession and measuring the NMR responses between the electromagnetic pulses. The measured NMR responses provide useful information about the sample volume.
NMR measurements may be used to detect, for example, the abundance of hydrogen contained within an underground sample volume, and NMR relaxation times within a sample. Detected hydrogen abundance and NMR relaxation times may be used to characterize many properties of fluid-bearing formations underground, such as the porosity, total quantity of fluids, fluid composition, fluid viscosity, pore size, wettability, and permeability of the sample. This disclosure is therefore directed to applications of NMR to detect water and hydrocarbons during induced alteration processes.